Cold shutdown that doesn't require coolant circulation?

[etudiant]

Decay heat can be somewhat mitigated by operating a reactor at low power density with fuel to low burnup (as is planned in at least one SMR design, and to some extent in a CANDU), but then there is an economic penalty.

Very interesting information.
It strongly suggests that CANDU designs are inherently safer.
How large is the 'economic penalty' you indicate?
Could the safety differential justify that difference?

 

[nikkkom]

Very interesting information.
It strongly suggests that CANDU designs are inherently safer.
How large is the 'economic penalty' you indicate?

More frequent fuel reloading and more voluminous waste. Something like x3 more waste by mass, but which is about x3 less radioactive.

 

[gmax137]

Very interesting information.
It strongly suggests that CANDU designs are inherently safer.
...

I'm not so sure about that. The decay heat level in the first hours following the reactor shutdown/trip are barely affected by the burnup (for any reasonable burnup). And, I think that the most risk occurs during those early hours, because it seems that the likelihood of core melt is much less at longer times, when decay heat is lower and more operator action (including aid from offsite) is possible.

In other words, lower burnup reduces the decay heat in the long term (days after trip), but that isn't where the big problems are.

 

[rmattila]

I'm not so sure about that. The decay heat level in the first hours following the reactor shutdown/trip are barely affected by the burnup (for any reasonable burnup). And, I think that the most risk occurs during those early hours, because it seems that the likelihood of core melt is much less at longer times, when decay heat is lower and more operator action (including aid from offsite) is possible.

In other words, lower burnup reduces the decay heat in the long term (days after trip), but that isn't where the big problems are.

Burnup does indeed not have a big effect, but power density wrt total heat capacity in the core does. CANDU, RBMK and AGR are good in this respect but have other, less favourable characteristics in other fields.

 

[etudiant]

The greater volume of spent fuel is clearly an economic issue.
Is the more frequent refuelling of the CANDU also an issue if the reactor can be refuelled during ongoing normal operations?

 

[Astronuc]

The greater volume of spent fuel is clearly an economic issue.
Is the more frequent refuelling of the CANDU also an issue if the reactor can be refuelled during ongoing normal operations?

CANDU units can do on-line refueling, so they can maintain high capacity factors. The burnups have been in the range of 1-1.5% FIMA, but may now be higher. The enrichments are lower, so the utility does not have to purchase more uranium ore as compared to LWRs using higher enrichment, which partially offsets the increased volume of spent fuel.

 

[mheslep]

True, BWR water is radioactive

Just curious: that's due only to the tritium atoms in the water? Not another source?

 

[Hiddencamper]

Just curious: that's due only to the tritium atoms in the water? Not another source?

While the fuel in BWRs (and PWRs) is solid, all solid material has some miniscule amounts of diffusion. As such, some fission products get into the primary coolant, such as Iodine, Cesium, Xenon, and even Boron from the control rods. During normal operation, there are chemistry samples done, and the specific activity of all of these fission products are looked at, as the ratio of the different fission product decay chains is a sign of whether or not the fuel has failed (Cracked) or if it is just simple diffusion of fission products through the cladding material.

Tritium comes not just from hydrogen absorbing neutrons, but also from the boron in the control rods. The B-10 can absorb a neutron and then undergo double alpha decay, leaving behind a tritium atom. Any boron in primary coolant, or any tritium/boron that leaches out of the rods will also increase tritium inventory in the primary coolant.

In all reactors, when the reactor is online, the main source of radiation in the primary coolant loop is N-16. N-16 is a very short lived isotope (several seconds), and is virtually completely gone within a few minutes after shutdown. When the reactor is offline, cobalt-60 (which comes from stellite material in valve seats as well as on control rod blade rollers used for preventing the blades from rubbing the fuel material), Co-60 is the main gamma emitter when the reactor is offline, usually in the form of hot particles which get trapped in the reactor coolant system.tl;dr most of the fission products and decay chains make it into primary coolant, not just tritium.

Additionally, primary coolant in both BWRs and PWRs is radioactive. PWRs have more tritium because they use Boron as a chemical shim, while the only tritium in BWR coolant is that from neutron capture and leeching. BWRs do not have a secondary coolant loop, but PWRs do, and their secondary loop also has radioactive products in it. PWRs have drastically less, as only things which leech through the steam generator tubes or pass through tube leaks generally get into secondary coolant. Additionally, reclaimed rad-waste water (which is reprocessed for reactor or secondary use) may contain slight amounts of fission products which weren't removed in the rad waste system. Secondary cooling loops have rather large levels of tritium however (compared to BWRs) as well, because tritium does not get removed in the normal rad waste process, as it chemically looks the same as normal water, and rad waste processing is primarily chemical/resin/ion exchange based.

 

[mheslep]

In all reactors, when the reactor is online, the main source of radiation in the primary coolant loop is N-16. N-16 is a very short lived isotope (several seconds),

Interesting. Which comes about from dissolved N2 gas in the water, or some nitrate hanging about?

 

[Hiddencamper]

Interesting. Which comes about from dissolved N2 gas in the water, or some nitrate hanging about?

It is an (n,p) reaction:

O16 + n -> N16 + p

The oxygen is from the water in the reactor vessel.

See http://en.wikipedia.org/wiki/Nitrogen

N-16 is the reason we have a 3 foot thick concrete bioshield around BWR heater bays and turbines.

 

[mheslep]

It is an (n,p) reaction:

O16 + n -> N16 + p

The oxygen is from the water in the reactor vessel.

Ah of course, I should have seen that.

Continuing, the fuel itself is an oxide. I would think that would create problems, rapidly braking the oxide bonds of the fuel in the conversion of O to N.

 

[Hiddencamper]

Ah of course, I should have seen that.

Continuing, the fuel itself is an oxide. I would think that would create problems, rapidly braking the oxide bonds of the fuel in the conversion of O to N.

The fuel pellet is pretty much lost the moment you do your first heatup on the fuel. It's known to expand, crack, and under some very nasty transients or against heat limits, shatter/vaporize. Over time, due to changes in the composition of the fuel pellet itself, and changes in the cladding, your thermal limits become more limiting and your heat transfer rates get reduced. These are all accounted for in both core design and core modelling, and are validated in real time against actual plant data.

 

[nikkkom]

Just curious: that's due only to the tritium atoms in the water? Not another source?

There is little tritium in BWRs, since they have almost no deuterium, and produce tritium by other means than D+n->T. Tritium production is only significant in heavy water reactors.

While the fuel in BWRs (and PWRs) is solid, all solid material has some miniscule amounts of diffusion.

Not only that. A large BWR contains on the order of 50 thousands of individual fuel rods. With such a large number of rods, it's impractical to ensure that absolutely all of them stay watertight. Thus, BWRs are not stopped when tests indicate that just one single rod ruptured and water is now in touch with its fuel ceramic pellets, washing out some fission products.

 

[Hiddencamper]

There is little tritium in BWRs, since they have almost no deuterium, and produce tritium by other means than D+n->T. Tritium production is only significant in heavy water reactors.
Not only that. A large BWR contains on the order of 50 thousands of individual fuel rods. With such a large number of rods, it's impractical to ensure that absolutely all of them stay watertight. Thus, BWRs are not stopped when tests indicate that just one single rod ruptured and water is now in touch with its fuel ceramic pellets, washing out some fission products.

Reactor water chemistry is regularly sampled for the difference between diffusion, and actual leakage/seepage/cracking of the fuel. Once ratios of specific elements like iodine and xenon are seen to go outside of normal, in a BWR you can perform suppression testing. What we've found is if you push control rods in near the suspected leakers, you will see a decrease in radioactive inventory in the reactor coolant system. If you then push in 1 or 2 face adjacent controls rods and possibly a diagonal rod it will greatly suppress the amount of leakage from the leaky bundle, almost returning it to 'normal' levels for the reactor. You can then continue operating the unit, albeit with lost effective full power days.

In a PWR, a fuel leak almost always requires the fuel be removed and replaced. PWRs cannot run with a rod full into suppress it the way a BWR can.

 

[mheslep]

It is an (n,p) reaction:

O16 + n -> N16 + p

...

BTW, what happens to the continuously generated hydrogen, the H2 left behind (and the p when it neutralizes)?

 

[rmattila]

BTW, what happens to the continuously generated hydrogen, the H2 left behind (and the p when it neutralizes)?

In a BWR, non-condensible gases end up in the condenser vacuum system, recombiners to recombine most O2 and H2 back to water, then to the off-gas system to be delayed and filtered, and eventually to the atmosphere through the stack.

Hydrogen has a nasty habit of moving with steam in the primary piping and accumulating in places where steam condenses (e.g. inside certain valves), causing fragility issues with certain steel materials.

 

[Hiddencamper]

In a BWR, non-condensible gases end up in the condenser vacuum system, recombiners to recombine most O2 and H2 back to water, then to the off-gas system to be delayed and filtered, and eventually to the atmosphere through the stack.

Hydrogen has a nasty habit of moving with steam in the primary piping and accumulating in places where steam condenses (e.g. inside certain valves), causing fragility issues with certain steel materials.

Another note about this is BWRs usually inject hydrogen into their water to help protect the core and vessel from oxidation. This has some unpleasant side effects like increased radiation rates, fouling of venturis and instrument lines, and plating out of materials (could be good or bad), but is all in all beneficial for the plant as it prevents certain types of stress corrosion cracking.

 

[HowlerMonkey]

So...you guys think that a convective loop could be constructed that could deal with a recently shut down or scrammed reactor or do you know of a reactor design of similar power to current reactors that could be shut down and not need continuous power to run cooling pumps?

 

[rmattila]

So...you guys think that a convective loop could be constructed that could deal with a recently shut down or scrammed reactor or do you know of a reactor design of similar power to current reactors that could be shut down and not need continuous power to run cooling pumps?

That design gets pretty close:

http://www.rosatom.ru/wps/wcm/connect/spb_aep/site/resources/f3b59380478326aaa785ef9e1277e356/AES-2006_2011_EN_site.pdf

 

[QuantumPion]

So...you guys think that a convective loop could be constructed that could deal with a recently shut down or scrammed reactor or do you know of a reactor design of similar power to current reactors that could be shut down and not need continuous power to run cooling pumps?

The GE ESBWR is a boiling water reactor that operates by natural convection. The emergency core cooling system is a huge gravity-fed water tank that can keep the core cool with no offsite power or operator intervention for 3 days, after which it only requires replacing the water inventory at atmospheric pressure.

 

[HowlerMonkey]

Really glad to see that there are designs that won't "go up" from loss of electrical power or fuel to run diesel generators and pumps.

 

[mudweez0009]

I see some mention about cooldown limits of the RCS, which brings up a question.
I take it that when ALL power is lost (normal and Emerg.), the steam/turbine driven AFW pumps are used to deliver AFW to the S/G, and the RCS will be cooled in order to let borated water be injected from the refueling water storage tank (or VCT?), but the concern of RCS cooldown is brittle fracture, so that's why it is stopped at a certain point. Brittle fracture wasnt mentioned in this thread so I was wondering if this was true. I read about it in the Westinghouse Technology Systems Manual (Section 3.2).

Also, in terms of a tube rupture, when the RCS is cooling, I imagine that the cooldown is useful to allow the RCS to de-pressurize (in order to help prevent further coolant leakage).. Is this correct?

edit: This is in reference to PWRs

 

[Hiddencamper]

There is little tritium in BWRs, since they have almost no deuterium, and produce tritium by other means than D+n->T. Tritium production is only significant in heavy water reactors.



Not only that. A large BWR contains on the order of 50 thousands of individual fuel rods. With such a large number of rods, it's impractical to ensure that absolutely all of them stay watertight. Thus, BWRs are not stopped when tests indicate that just one single rod ruptured and water is now in touch with its fuel ceramic pellets, washing out some fission products.

In a BWR, any small leaks are, for the most part, caught and decayed in the condenser off-gas system.

If there is an increase in rad-levels in the offgas system, then the system will isolate and an alarm will go off. chemistry will perform sampling to confirm a fuel leak based on iodine/xenon ratios. Based on how bad the leak is, there is a criteria for what is allowable. If you do not exceed that, then operations will perform "Power Suppression Testing". For small leaks in a BWR, if you insert control rods near the fuel bundle, the reduction in fuel rod pressure/temperature will almost completely stop the fuel leak. If the location can be confirmed during testing, AND if the leak rate decreases back below the 'normal' limits, then that fuel cell, and the cells directly adjacent to it, will have their control rods inserted to suppress those cells and stop the leak. Operation can be continued through the end of the next operating cycle, however a single leaker tends to reduce cycle length by up to 10% or so (Obviously, the later you are in the cycle, the less of an impact there will be. leakers tend to happen during the first startup and preconditioning following a refuel).

At the next refuel cycle, the leakers will be confirmed by siping. The individual rods may be sent back to the fuel vendor for analysis. Leakers really suck, because now you have to deal with iodine in your systems, which makes a whole new set of radiation concerns whenever you have to breach a potentially contaminated system. Dose rates all over the plant shoot up, and areas that normally aren't rad or high rad areas will become high rad areas. You end up spending a lot of time during the next outage flushing pipes and installing shielding just to get dose rates in general areas down. It's sucky.

PWRs cannot operate with these types of leakers. If the leak rates are too bad, they cannot simply drop in 1 control rod assembly, as they will get flux tilt in excess of allowable limits. PWRs need to come offline for bundle replacement.

 

[QuantumPion]

PWRs cannot operate with these types of leakers. If the leak rates are too bad, they cannot simply drop in 1 control rod assembly, as they will get flux tilt in excess of allowable limits. PWRs need to come offline for bundle replacement.

PWR's do not go offline to replace leakers, they continue normal operation (unless the RCS activity levels go beyond allowable limits, which I've personally never heard of happening). Leakers usually show up after coming back online from a trip in the middle of the cycle. Most contamination is filtered out, but the primary systems will be more radioactive during the next outage. After the cycle is complete, we use sipping to identify the leaker. Typically they occur in the final fuel cycle for the assembly, in which case nothing special is done. If the leaker occurs in a 2nd cycle assembly, a replacement with equivalent burnup from the fuel pool is used. In the less likely event a 1st cycle assembly has failed, the assembly is reconstituted by replacing the leaking rod with a stainless steel filler rod.

Note that PWRs can operate with a dropped rod, depending on the specifics of the unit. I remember some years ago we redid our safety analysis to allow the unit to continue to end of cycle with a dropped rod late in the cycle - it was doable because late in the cycle peaking factors are low enough and rod worth high enough to meet all safety parameters. However you are correct that PWR's do not intentionally insert a single rod for any purpose :shy:

 

[Hiddencamper]

Ok makes sense.

One of our PWRs had a leaker and came off about 5 years ago. But obviously not every leak is the same, and PWRs have a lot of variation.

As for drop rod, the same PWR in my company only allows a rod drop for a few hours, and if it can't be fixed, they need to trip.

 

[QuantumPion]

Ok makes sense.

One of our PWRs had a leaker and came off about 5 years ago. But obviously not every leak is the same, and PWRs have a lot of variation.

As for drop rod, the same PWR in my company only allows a rod drop for a few hours, and if it can't be fixed, they need to trip.

Yes that's the tech specs but it is possible to re-do the safety analysis to accommodate the dropped rod to resume operation with it still stuck in without having to go into refueling.

 

[nikkkom]

Thanks, very informative!

At the next refuel cycle, the leakers will be confirmed by siping. The individual rods may be sent back to the fuel vendor for analysis.

Are you saying BWR plant personnel can remove a spent fuel assembly and remove individual rods from it?

I thought that spent fuel, especially freshly unloaded one, sits in the pool for a few years as a minimum before anything is done to it.

 

[QuantumPion]

Thanks, very informative!



Are you saying BWR plant personnel can remove a spent fuel assembly and remove individual rods from it?

I thought that spent fuel, especially freshly unloaded one, sits in the pool for a few years as a minimum before anything is done to it.

A whole fuel bundle must cool for 5 years before it can be moved to dry storage. But individual rods can be shipped out for analysis as the decay heat load of a single rod is only a fraction of the total.

 

[gmax137]

...I thought that spent fuel, especially freshly unloaded one, sits in the pool for a few years as a minimum before anything is done to it.

Individual rods can be removed from the spent assemblies; the work is done with special tools while the assembly remains submerged in the pool. AFAIK, this is kind or rare for PWR fuel, but it can be done.

 

[Astronuc]

Thanks, very informative!


Are you saying BWR plant personnel can remove a spent fuel assembly and remove individual rods from it?

I thought that spent fuel, especially freshly unloaded one, sits in the pool for a few years as a minimum before anything is done to it.

Modern LWR fuel is 'reconstitutable', i.e., the upper nozzles or tie plates can be removed and the fuel rods removed, and examined. Some fuel rods are removed for various measurements.